Recent research suggests that mammals have been nocturnal, living it up under the cover of night, for longer than they have been truly mammalian.
The sclerotic ring of a gorgonopsid, a carnivorous mammal-like reptile. Photograph: Kenneth Angielczyk
Many species of mammal are active during the night, leading scientists to believe that nocturnality is an ancient mammalian trait.
“Nocturnality is widespread among extant [currently living] mammals and often considered the ancestral behavioural pattern for all mammals,” write the authors of the paper, published in the journal Proceedings Of The Royal Society B.
‘Which mammals are nocturnal?’ You might ask. Many species of cat, big and small, hunt during the night – making them less visible to prey. Many rodents use the cover of night for the same reason, but for different ends – making themselves less visible to predators. Bats are also obvious nocturnal mammals.
‘Big deal’ you might say, ‘cats, rats, and bats’. That is until you’re told that rodents make up 40% of all mammal species, and bats, 20%. A huge proportion of mammals are nocturnal.
It was previously thought that mammals evolved nocturnality when they first came on to the scene 200 million years ago. Since mammals evolved only once dinosaurs were established, nocturnality may have been an adaptation to avoid the ‘terrible lizards’.
The study published on the 3rd of September, analysed fossils from a group of reptiles called synapsids. Synapsids were around long before mammals, yet had many mammalian characteristics. Analysing the fossil record from older rock to newer rock, we see a gradual transition from synapsid reptile to proto-mammal to mammal. Coupled with the family resemblance, this suggests that some synapsid reptiles were the direct ancestors of mammals – and that means you. Imagine a distant grandparent, fearsome and powerful, defending its territory long before dinosaurs had even evolved.
Along with birds and lizards, the synapsids had bones in their eye sockets which steadied their eye balls, kind of like an egg cup. This bone is called the sclerotic ring. The structures of the sclerotic ring and the eye socket reveal characteristics of the creatures’ eyes, including how big they were. And since eye size is related to the amount of light available to the creature, scientists can determine the times of day when the creatures were most active.
By studying the fossilised eye characteristics of 24 species of these reptilian synapsids, the researchers discovered that many of the creatures were nocturnal. This suggests that synapsids were hunting nocturnally before mammals even existed. And since a small group of synapsids eventually evolved into mammals, the traits for night living were passed down. By using these ancient traits for night-time living, mammals have branched off into all the nocturnal diversity we see today.
A nocturnal feeding scene between mammal-like reptiles, dimetrodons. REUTERS/Marlene Hill Donnelly/Handout via Reuters
Research paper: Angielczyk, K.D., and Schmitz, L. (2014) Nocturnality in synapsids predates the origin of mammals by over 100 million years. Proc. R. Soc. B, 281 (1793) doi:10.1098/rspb.2014.1642
Originally published on Thinkinc.org.au
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